The present invention relates to a display device employing a field-sequential method for displaying a color image by synchronizing the light-emission timing of each color of emitted light and the switching of a light switching element for controlling the intensity of light for display.
Along with the recent development of so-called information-oriented society, electronic apparatuses, such as personal computers and PDA (Personal Digital Assistants), have been widely used. Further, with the spread of such electronic apparatuses, portable apparatuses that can be used in offices as well as outdoors have been used, and there are demands for small-size and light-weight of these apparatuses. Liquid crystal display devices have been widely used as one of the means to satisfy such demands. Liquid crystal display devices not only achieve small size and light weight, but also include an indispensable technique in an attempt to achieve low power consumption in portable electronic apparatuses that are driven by batteries.
By the way, the liquid crystal display devices are mainly classified into the reflection type and the transmission type. In the reflection type liquid crystal display devices, light rays incident from the front face of a liquid crystal panel are reflected by the rear face of the liquid crystal panel, and an image is visualized by the reflected light; whereas in the transmission type liquid crystal display devices, the image is visualized by the transmitted light from a light source (back-light) provided on the rear face of the liquid crystal panel. Since the reflection type liquid crystal display devices have poor visibility resulting from the reflected light amount that varies depending on environmental conditions, transmission type liquid crystal display devices are generally used as display devices of, particularly, personal computers displaying a multi-color or full-color image.
In addition, the current color liquid crystal display devices are generally classified into the STN (Super Twisted Nematic) type and the TFT-TN (Thin Film Transistor-Twisted Nematic) type, based on the liquid crystal materials to be used. The STN type liquid crystal display devices have comparatively low production costs, but they are not suitable for the display of a motion image because they are susceptible to crosstalk and comparatively slow in the response speed. In contrast, the TFT-TN type liquid crystal display devices have better display quality than the STN type, but they require a back-light with high intensity because the light transmittance of the liquid crystal panel is only 4% or so at present. For this reason, in the TFT-TN type liquid crystal display devices, a lot of power is consumed by the back-light, and there would be a problem when used with a portable battery power source. Moreover, the TFT-TN type liquid crystal display devices have other problems including a low response speed, particularly, in displaying half tones, a narrow viewing angle, and a difficult color balance adjustment.
Therefore, in order to solve the above problems, the present inventors et al. are carrying out the development of liquid crystal display devices using ferroelectric liquid crystals or antiferroelectric liquid crystals having spontaneous polarization and a high response speed of several hundreds to several xcexcs order with respect to an applied voltage. When a liquid crystal material having spontaneous polarization, such as ferroelectric liquid crystal and antiferroelectric liquid crystal, is used as the liquid crystal material, the liquid crystal molecules are always parallel to the substrate irrespective of the presence or absence of applied voltage, and the change in the refraction factor in the viewing direction is much smaller compared with the conventional STN type and TN type. It is thus possible to obtain a wide viewing angle.
Furthermore, the present inventors et al. who are carrying out the research of a liquid crystal display device that drives such a liquid crystal material having spontaneous polarization by a switching element such as a TFT have developed a liquid crystal display device employing a field-sequential method, which uses ferroelectric liquid crystal elements or antiferroelectric liquid crystal elements having a high response speed to an applied electric field as the liquid crystal elements and displays a color image by causing a single pixel to emit light of three primary colors in a time-divided manner. Such a liquid crystal display device realizes a color display by combining a liquid crystal panel using ferroelectric liquid crystal elements or antiferroelectric liquid crystal elements capable of responding at a high speed of several hundreds to several xcexcs order with a back-light capable of emitting red, green and blue lights in a time-divided manner and by synchronizing the switching of the liquid crystal element with the light emission of the back-light, more specifically, by dividing one frame into three sub-frames and causing a red LED, a green LED and a blue LED to emit light in the first sub-frame, the second sub-frame and the third sub-frame, respectively.
A display device employing a field-sequential method as described above can easily display a more definite image compared with a display device employing a color-filter method, and has advantages such as high brightness, excellent purity of display color, high light utilization efficiency and low power consumption because it uses the light emission of the light source as it is for display without using a color filter. In the display device employing a field-sequential method, however, since an image is displayed by switching the colors of light emitted by the light source, such as red, green and blue, the images of three colors having a time difference are not superimposed on the same point on the retina of a human when he/she moves the line of sight, and therefore there is a problem of occurrence of a phenomenon called xe2x80x9ccolor breakxe2x80x9d in which a display color different from the original image is momentarily recognized.
A principal object of the present invention is to provide a display device employing a field-sequential method, capable of reducing color break without considerably changing the power consumption and the displayable temperature range.
A display device of the first aspect is a display device employing a field-sequential method for displaying a color image by sequentially switching a plurality of colors of emitted light of a light source within one frame and by synchronizing a light-emission timing of each color of emitted light with a switching of a light switching element for controlling an intensity of light for display, and comprises changing means for changing a frame number per unit time.
According to the first aspect, a reduction of color break is achieved by changing the frame number per unit time in displaying a color image by synchronizing the light-emission timing of the color of emitted light with the switching of the light switching element for controlling the intensity of light for display. Color break is caused by the movement of the line of sight of the user and the time-lapse display of display colors. Therefore, by shortening the switching time of the color of emitted light, i.e., by increasing the frame number per unit time, it is possible to reduce color break. However, when a reduction of color break is made in such a manner, problems arise that the displayable temperature range is narrowed and the power consumption increases with an increase of the frame number. Then, in the first aspect, by changing the frame number per unit time according to a condition, i.e., by increasing the frame number when color break is noticeable or decreasing the frame number when color break is not noticeable, a reduction of color break is achieved without considerably changing the displayable temperature range and the power consumption.
A display device of the second aspect is based on the first aspect, wherein the changing means comprises discriminating means for judging whether display data is motion picture data or still picture data, and means for changing the frame number per unit time based on the result of the judgement by the discriminating means.
According to the second aspect, the frame number is changed based on the type of display data (motion picture data or still picture data). In displaying a motion image in which the user moves the line of sight, color break occurs noticeably. Therefore, by changing the frame number in displaying a motion image and in displaying a still image, it is possible to reduce color break efficiently according to the type of the display data.
A display device of the third aspect is based on the second aspect, wherein, when the display data is motion picture data, the frame number per unit time is increased compared with the frame number for still picture data.
In the third aspect, the frame number is increased for the display of a motion image during which color break easily occurs, while the frame number is made smaller than that for the display of a motion image for the display of a still image during which color break hardly occurs. Accordingly, it is possible to reduce color break without causing a significant increase in the power consumption
A display device of the fourth aspect is based on the first aspect, wherein the changing means comprises detecting means for detecting the temperature of the light switching element, and means for changing the frame number per unit time based on the result of the detection by the detecting means.
In the fourth aspect, the frame number is changed based on the temperature of the light switching element. When the frame number is increased so as to reduce color break, the time of each sub-frame is shortened, and therefore, if a liquid crystal display element is used as the light switching element, the responsiveness of the liquid crystal is lowered due to an increase in the viscosity of the liquid crystal caused by a decrease of the temperature although the liquid crystal is required to have a fast responsiveness. For this reason, when the frame number is increased, in general, it becomes difficult to display an image on a low-temperature side, resulting in a narrower displayable temperature range. Therefore, by changing the frame number for a high temperature state or for a low temperature state, it is possible to reduce color break efficiently according to the temperature state.
A display device of the fifth aspect is based on the fourth aspect, wherein, when the temperature of the light switching element is higher than a predetermined temperature, the frame number per unit time is increased compared with the frame number for a temperature lower than the predetermined temperature.
In the fifth aspect, at high temperature at which there is no possibility of display difficulty, the frame number is increased so as to reduce color break, while at low temperature at which there is a possibility of display difficulty, the frame number is decreased so as to enable display that has priority over the reduction of color break. Accordingly, it is possible to achieve both the reduction of color break at a frequently used high-temperature range and the retention of the displayable temperature range, thereby reducing color break without narrowing the displayable temperature range.
A display device of the sixth aspect is based on any one of the first through fifth aspects, wherein the light switching element is a liquid crystal display element.
In accordance with the sixth aspect, a liquid crystal display element is used as the light switching element, and it is possible to reduce color break in the liquid crystal display.
A display device of the seventh aspect is based on the sixth aspect, wherein the liquid crystal display element includes a liquid crystal material having spontaneous polarization.
In accordance with the seventh aspect, since a liquid crystal material having spontaneous polarization is used in the liquid crystal element, it is possible to obtain a wide viewing angle.
A display device of the eighth aspect is based on the sixth or seventh aspect, wherein the liquid crystal display element comprises an active element corresponding to each of a plurality of liquid crystal pixels.
In accordance with the eighth aspect, in the liquid crystal display element, since each of a plurality of liquid crystal pixels is independently controlled and driven by the active element, it is possible to obtain high display characteristics.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.